The invention pertains to microlenses, and in particular embodiments, to microlenses and methods of making microlenses having a refractive index gradient for providing desired refractive properties.
Microlenses are employed for collecting, focusing and steering light. Conventional microlens devices typically comprise an array of miniature spherical lenses that are formed on a substrate using conventional semiconductor processing techniques. FIGS. 1a-1c illustrate a conventional microlens formation process. In FIG. 1a, a layer of a lens material 20 such as silicon dioxide is formed on glass substrate 10. An array of photoresist cylinders 30 is formed over the layer of lens material. The photoresist 30 is then heated and reflowed, producing reflowed photoresist bodies 40 as illustrated in FIG. 1b. The surfaces of the bodies 40 formed in this are virtually spherical. Etching is then performed using a conventional etching process, such as reactive ion etching, that is particularly chosen to provide an approximately 1:1 photoresist to lens material etch ratio. As a result producing an array of spherical microlenses 50 is produced as illustrated in FIG. 1c. These conventional techniques are described in Z D Popovic, R A Sprague, G A N Connell, xe2x80x9cTechnique for monolithic fabriation of microlens arraysxe2x80x9d, Appl. Opt., 27, 1281-1284 (1988), and in D Daly, R F Stevens, M C Hutley, N Davies, xe2x80x9cThe manufacture of microlenses by melting photoresistxe2x80x9d, J Meas. Sci., 1, 759-766 (1990).
A disadvantage of spherical lenses is that they are not capable of aberration free focusing, that is, focusing light at a single point. FIG. 2 illustrates the convergence of rays of light exiting the flat base surface of a spherical lens. As seen in FIG. 2, rays 60 exiting from the outermost edges of the lens converge at a point 62 that is higher than the point of convergence 72 of rays 70 that exit the center of the lens. The narrowest spot size that can be produced by the lens occurs at the plane indicated by line Axe2x80x94A. Even at this plane, only a fraction of the refracted light is fully converged.
Variations on the conventional technique may be employed to produce lenses having aspherical cross-sections. For example, aspherical lenses can be formed by gray scale photolithogaphy. Aspheric lenses can also be formed by using a variation of the conventional photoresist reflow-and-etch process. In this method, the etch conditions are varied during etching, such that the spherical photoresist body produces an aspherical body in the substrate material.
These shaping techniques are limited in the degree of precision with which they can shape a lens surface, and consequently they are not able to tailor microlens refractive properties with the accuracy that would be desirable for current microlens applications. However, formation of spherical lenses by the melting or reflow is an easy and reliable technique and is desirable for those reasons.
Embodiments of the invention overcome disadvantages of conventional microlenses and microlens fabrication techniques by employing a lens material having a refractive index that varies along its z-axis and shaping the microlens structure from such material by using, for example, conventional microlens shaping techniques. The gradient of the refractive index along the z-axis is designed to provide desired refractive properties in conjunction with the lens surface shape. Accordingly, the refractive properties of conventionally shaped microlenses are controlled with great precision through design and manipulation of the refractive properties of the lens material.
Embodiments of the invention relate to a method for designing and fabricating a microlens, wherein a lens surface shape is defined, desired refraction properties of the lens are defined, and a material is formed that has a refractive index gradient in the direction of the z-axis that is determined based upon the desired refraction properties and the defined surface shape of the lens. Further embodiments of the invention relate to a method for fabricating a microlens, wherein a material gradient providing a desired refractive index gradient is determined based upon a lens surface shape and desired refractive parameters, a graded index material having the material gradient is deposited, and a microlens having the desired surface shape is formed in the graded index material. Further embodiments of the invention relate to a microlens structure having a refractive index gradient in the direction of the z-axis for providing predetermined lens refractive properties in conjunction with the lens surface shape.
In accordance with an exemplary embodiment of the invention, the lens material may be deposited in a continuous deposition process, in which processing parameters such as deposition gas pressures or flow/evaporation rates are varied continuously to vary the composition of the deposited material layer and thereby vary the refractive index. Such graded refractive index thin films may be deposited with precision control for a wide range of materials by physical (i.e. evaporation) and chemical vapor deposition (e.g., MOCVD) techniques. Alternatively, the lens material may be comprised of discrete sub-layers having different refractive indices.
The lens material is chosen in accordance with the wavelength of light for which the lens is to be applied. Preferred embodiments of the invention for visible wavelength applications may employ silicon oxides SiOX or combinations of SiO and TiO2. It is desirable to employ materials that provide a 1:1 etching ratio using a known etching technique.
Embodiments of the invention enable formation of lenses in accordance with desired refractive properties. A lens may be designed in accordance with an example embodiment of the invention by determining a desired lens shape, determining a refractive index gradient that will provide the desired convergence, and determining a lens material gradient that will provide the desired refractive index gradient. The refractive index gradient may be determined iteratively to minimize the error of simulated refractive properties relative to ideal desired refractive properties. A lens may be fabricated in accordance with an example embodiment of the invention by forming a lens material layer having a material gradient in the direction of the z-axis in accordance with a design as described above, and etching the material to form microlenses having a desired surface shape.
Microlenses in accordance with the invention have a wide range of applications such as beam steering, coupling, and image display.